The strength of the earthquake which left much of Christchurch in rubble on Tuesday may have been magnified as it reflected off hard volcanic rock below its epicentre, an earthquake geologist says. Tuesday's quake was part of the aftershock sequence which followed September's magnitude 7.1 quake near Darfield, 40km west ofthe city, GNS Science earthquake geologist Kelvin Berryman said today. Mr Berryman said part of the reason why the quake was felt so strongly could have been because it reflected off volcanic rock near its epicentre. Seismic energy travelled in waves and could be reflected off hard surfaces, much like sound waves, he said. ``With the epicentre of Tuesday's earthquake in the Port Hills, a large amount of energy could have been reflected off hard volcanic rock at depth. This would have compounded the impact of the earthquake at the surface''. He said there had been an unusual number of aftershocks following Tuesday's quake but these were beginning to weaken and become less frequent but warned there were often anomalies, ``as the magnitude 6.3 earthquake had shown''. The quake was part of an expanding ``cloud'' of aftershocks which was a familiar pattern with large earthquakes following quakes of similar magnitude to September's 7.1 magnitude earthquake, he said. ``Aftershocks have been spreading both west and east since the magnitude 7.1 Darfield earthquake in September,'' he said.
